In a longwall mining installation, coal is removed across a face area by a longwall mining machine having forward and aft cutters. As coal is mined and removed, it is deposited onto a conveyor which extends along the longwall face. Hydraulically-powered, self-advancing roof supports or chocks protect equipment and personnel in the working area. As mining advances and the supports are moved forward, the roof behind the supports caves.
A large number of side-by-side roof supports is spaced along the face being mined. Each support comprises a base, a canopy which engages the mine roof, and generally vertical hydraulic cylinders which extend between the base and the canopy and are extended between the base and canopy to exert pressure on the mine roof and prevent it from collapsing. After the longwall mining machine has passed along the face, the upward pressure on the canopy of each support is released. Thereafter, generally horizontal hydraulic cylinders on the individual roof supports or chocks push the face conveyor forward and then pull themselves up into place over it by means of the same cylinders. At this point, upward pressure on the canopy is again exerted to hold the mine roof in place; while the roof behind the advanced support is permitted to fall.
In the past, it has been common to manually control each individual roof support along the face being mined by means of valves. However, some systems have been devised which employ a remote control process wherein the individual roof supports are connected to and controlled by a central dispatcher control system. Each individual roof support is provided with an individual address or identifying signal which is sent from the dispatcher control system. Each support, in turn, is able to detect its identifying signal or address whereby control signals for achieving certain desired control functions can be effected at any individual support. For example, addressing can be achieved by pulse transmission at different frequencies and by signal selection by filters associated with a particular support, whereby direct connection of the individual roof supports to the dispatcher control can be achieved via an electrical multi-conductor installation. Direct connection, however, requires a very large number of individual conductors.
Remote and automatic control of individual roof supports for a longwall mining installation have been provided in which face support control is effected by means of a microprocessor system, using the position of the longwall machine to control individual roof supports ahead of and behind the path of travel of the mining machine. In a system of this type utilizing a microprocessor, pulses proportional in number to the distance covered by the longwall mining machine are transmitted to a central control panel disposed in the roadway or at the end of the face. In this manner, the position of the mining machine along the face can be determined and control of roof supports ahead of and behind the mining machine effected. In one particular microprocessor control system of this type, a 16-bit data word is transmitted serially from a central control panel to control units on all of the individual roof supports. Contained in the data word is an identification of the roof support which is to be actuated and the specific control functions to be accomplished such as advancing the conveyor toward the face, lowering the roof support, pulling the lowered roof support toward the conveyor, and so on. In each control unit for the separate roof supports, means are included for comparing a transmitted identification of a particular roof support from the central panel to stored data in the individual control unit for that support; and if the two are the same, the control unit for an individual support transmits the received control information back to the central control panel for confirmation before a control command is executed and sent back to the individual roof support.
If individual roof supports are to be controlled manually as well as automatically from a central control panel, difficulties arise with the prior art methods of automatically controlling the roof supports from such a central control panel. Manual operation is necessary in the event of malfunctioning occurring at any particular roof support outside the immediate vicinity of the mining machine while mining operations continue. That is, it must be possible to check the functioning of the hydraulic cylinders of a roof support or to check the sequence of movement produced by the cylinders of a malfunctioning support and carry out any necessary repairs without interfering with the automatic control of the remaining roof supports.